1. Field of the Invention
The present invention relates to a polyimide composite material and a process for producing the same, and more particularly, it is concerned with a polyimide composite material which is composed of a polyimide-containing resin (hereinafter referred to as a polyimide resin) and a layered clay mineral dispersed therein and has improved water and gas barrier properties, and also with a process for producing the same.
2. Description of the Related Art
Polyimide is suitable for use as film, flexible printed circuit board, motor insulator, wire covering, etc. on account of its outstanding thermal properties, mechanical properties, electrical insulating properties, and chemical resistance. However, it poses a problem associated with practical use because of its poor gas (and water vapor) barrier properties and high coefficient of thermal expansion.
This problem has been addressed by introducing a fluorinated monomer into the molecule (as disclosed in Japanese Patent Laid-open Nos. 60933/1990 and 60934/1990) or by incorporating an inorganic substance (as such) into polyimide. To address a similar problem involved in polyamide resins, there has been proposed a composite material composed of a polyamide resin and a layered clay mineral dispersed therein which has previously been intercalated with an organic compound (U.S. Pat. No. 4,739,007).
The approach to solution by the aid of fluorinated monomer is not of practical value because of its high price. The mere incorporation of an inorganic substance (as such) into a polyimide resin ends up with incomplete dispersion due to their poor affinity, without achieving the improvement in gas barrier properties as intended, and a film formed of such composite material will lack the surface smoothness.
The technology of the polyamide composite material mentioned above cannot be applied to polyimide because of their difference in affinity for solvents. In other words, polyimide is produced by polymerization in a special solvent (i.e., aprotic polar solvent), and a clay mineral intercalated with organic compound usually has a weak affinity for such a special solvent. For this reason, a clay mineral does not disperse well in a polyimide.
There are disclosed in U.S. Pat. No. 4,775,586 composites and construction materials which are composed of a polyimide and a flocculant of intercalated clay formed by the reaction of a layered clay mineral with organic dionium ions. This patented invention is intended for flocculation rather than dispersion of intercalated clay into a polyimide resin. Therefore, this polyimide composite material does not have good gas and water barrier properties.